Power-assisted steering system

ABSTRACT

A power-assisted steering system includes a changeover valve unit comprising an input shaft to be rotated by the driver&#39;s steering effort applied thereto, an output shaft operatively connected to a hydraulic power cylinder associated with the steering system, a valve assembly for controlling the supply of fluid under pressure from a hydraulic pump to the power cylinder in response to relative rotation between the input and output shafts; and a hydraulic reaction mechanism for applying a reaction force to the input shaft in accordance with a hydraulic reaction pressure applied thereto. The reaction mechanism includes a reaction fluid chamber to be constantly applied with a predetermined small quantity of fluid under pressure and to be applied with a hydraulic pressure caused by relative rotation between the input and output shafts through a throttle. The system is arranged to increase the hydraulic pressure in the reaction fluid chamber in accordance with an increase of travel speed of the vehicle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power-assisted steering systems inautomotive vehicles, and more particularly to a power-assisted steeringsystem of the type which includes a hydraulic reaction mechanism forapplying a reaction force to the driver's steering effort in accordancewith travel speed of the vehicle.

2. Description of the Prior Art

In Japanese Patent Publication No. 49-30659 there has been proposed apower-assisted steering system of this kind which includes a hydraulicreaction mechanism for applying a reaction force to the driver'ssteering effort in accordance with a steering load acting on thedirigible road wheels of the vehicle and travel speed of the vehicle. Insuch a conventional steering system, the reaction mechanism includes areaction fluid chamber to be applied with a hydraulic pressure caused byoperation of a changeover valve unit in turning manueuvers of thevehicle. The hydraulic pressure in the reaction fluid chamber iscontrolled to decrease in accordance with an increase of travel speed ofthe vehicle. For this reason, when the changeover valve unit ismaintained in a neutral position, the hydraulic pressure in the reactionfluid chamber becomes approximately zero, resulting in unstability ofthe steering wheel in its neutral position.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to providean improved power-assisted steering system which is capable ofincreasing a reaction force applied to the steering wheel in its neutralposition in accordance with an increase of travel speed of the vehicle.

According to the present invention, the primary object is attained byproviding a power-assisted steering system which comprises a hydraulicpump driven by a prime mover of the vehicle for supply of fluid underpressure, a hydraulic power cylinder operatively connected to thedirigible road wheels of the vehicle, a changeover valve unit includingan input member arranged to be rotated by the driver's steering effortapplied thereto, an output member operatively connected to the powercylinder, and valve means for controlling the supply of fluid underpressure from the pump to the power cylinder in response to relativerotation between the input and output members, a hydraulic reactionmechanism associated with the changeover valve unit to apply a reactionforce to the input member in accordance with a hydraulic reactionpressure applied thereto, the reaction mechanism including a reactionfluid chamber to be applied with the hydraulic reaction pressure, afirst flow control valve disposed within a first fluid circuitconnecting the pump to the valve means of the changeover valve unit andwithin a second fluid circuit connecting the reaction fluid chamber to afluid reservoir to permit a predetermined small quantity of fluid underpressure supplied therethrough from the pump to the reaction fluidchamber, a first throttle disposed within a bypass circuit connectingthe first fluid circuit to the second fluid circuit between the firstflow control valve and the reaction fluid chamber, a second throttledisposed within the second fluid circuit between the first flow controlvalve and the fluid reservoir, and a second flow control valve disposedwithin the second fluid circuit downstream of the second throttle todecrease the quantity of fluid discharged therethrough into the fluidreservoir in accordance with an increase of travel speed of the vehiclethereby increasing the hydraulic reaction pressure in the reaction fluidchamber.

Alternatively, the primary object is attained by providing apower-assisted steering system which comprises a primary hydraulic pumpdriven by a prime mover of the vehicle for supply of fluid underpressure, an auxiliary hydraulic pump driven by the prime mover forsupply of a predetermined small quantity of fluid under pressure, ahydraulic power cylinder operatively connected to the dirigible roadwheels of the vehicle, a changeover valve unit including an input memberarranged to be moved by the driver's steering effort applied thereto, anoutput member operatively connected to the power cylinder, and valvemeans for controlling the supply of fluid under pressure from theprimary hydraulic pump to the power cylinder in response to relativemovement between the input and output members, a hydraulic reactionmechanism associated with the changeover valve unit to apply a reactionforce to the input member in accordance with a hydraulic reactionpressure applied thereto, the reaction mechanism including a reactionfluid chamber connected to the auxiliary hydraulic pump to be suppliedwith the predetermined small quantity of fluid under pressure, a firstthrottle disposed within a bypass circuit connecting a first fluidcircuit between the primary hydraulic pump and the valve means of thechangeover valve unit to a second fluid circuit between the auxiliaryhydraulic pump and the reaction fluid chamber, a second throttledisposed within a third fluid circuit connecting the second fluidcircuit to a fluid reservoir, and a flow control valve disposed withinthe third fluid circuit downstream of the second throttle to decreasethe quantity of fluid discharged therethrough into the fluid reservoirin accordance with an increase of travel speed of the vehicle therebyincreasing the hydraulic pressure in the reaction fluid chamber.

In a practical embodiment of the present invention, the hydraulicreaction mechanism may comprise a pair of opposed pistons axiallyslidably disposed with a radial bore in the output member located in thereaction fluid chamber, and a radial arm integral with the input memberand disposed between the pistons. Alternatively, the hydraulic reactionmechanism may comprise two pairs of opposed pistons axially slidablydisposed within a pair of parallel radial bores in the output memberlocated in the reaction fluid chamber, and a pair of radial armsintegral with the input member and disposed between each pair of thepistons. In addition, it is preferable that the second flow controlvalve is in the form of a solenoid relief valve disposed within thesecond fluid circuit to be energized by an electric control signalindicative of travel speed of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects, features and advantages of the present inventionwill be more readily appreciated from the following detailed descriptionof preferred embodiments thereof when considered with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of a power-assisted steering systemin accordance with the present invention;

FIG. 2 is a cross-sectional view taken along line 2--2 in FIG. 1,illustrating a hydraulic reaction mechanism in a changeover valve unitadapted to the steering system;

FIG. 3 is an enlarged sectional view of a flow control valve shown inFIG. 1;

FIG. 4 is a graph illustrating a relationship between a power cylinderpressure and a driver's steering effort; and

FIG. 5 is a schematic illustration of a modification of thepower-assisted steering system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 of the drawings there is illustrated a changeover valve unitof the rack and pinion type adapted to a power-assisted steering systemin anautomotive vehicle. The changeover valve unit includes a rotaryvalve assembly A for controlling the supply of fluid under pressure to ahydraulic power cylinder C associated with the steering system and ahydraulic reaction mechanism B for applying a reaction force to thedriver's steering effort in accordance with travel speed of the vehicle.The rotary valve assembly A comprises a valve housing 11, input andoutputshafts 13 and 14 arranged for relative rotation on aligned axeswithin the valve housing 11, a torsion bar 12 interconnecting the inputand output shafts 13 and 14, a valve rotor 13a integral with the inputshaft 13, and a valve sleeve 15 rotatably mounted within the valvehousing 11 in surrounding relationship with the valve rotor 13a. Thevalve sleeve 15 is connected with the output shaft 14 for rotationtherewith and cooperates with the valve rotor 13a to selectively supplyfluid under pressure to opposite fluid chambers C₁, C₂ of the hydraulicpower cylinder Cin response to relative rotation between the input andoutput shafts 13 and

The valve housing 11 is provided with an inlet port A₁ connected to adischarge port of a hydraulic pump P by way of a fluid circuit 21, anexhaust port A₂ connected to a fluid reservoir R of the pump P by wayofa fluid circuit 22, and a pair of ports A₃ and A₄ respectivelyconnectedto the opposite fluid chambers C₁ and C₂ of power cylinder C by way offluid circuits 23 and 24. The output shaft 14 is in the form of a pinionshaft permanently in mesh with a rack bar 16 which isintegrallyconnected to a power piston in the hydraulic power cylinder C ina usualmanner. The rack bar 16 is further operatively connected to a pair ofdirigible road wheels of the vehicle through a standard linkagemechanism (not shown). The hydraulic pump P is arranged to be driven bya prime nover of the vehicle.

When the valve rotor 13a is maintained in a neutral position, fluidunder pressure from the pump P is supplied to the inlet port A₁ of therotary valve assembly A through the fluid circuit 21 and is circulatedto the fluid reservoir R through the exhaust port A₂ and fluid circuit22. When relative rotation occurs between the input and output shafts 13and 14, the valve rotor 13a cooperates with the valve sleeve 15 toselectively supply fluid under pressure from the pump P to one of thefluid chambers C₁, C₂ in power cylinder C across one of the ports A₃, A₄and one of the fluid circuits 23, 24 and to permit the flow of fluiddischarged from the other fluid chamber C₂ or C₁ into the fluidreservoir R across the other fluid circuit 21 or 23, port A₄ or A₃, portA₂ and fluid circuit 22. This will provide a hydraulic power assist tothe driver's steering effort applied to the input shaft 13 duringturning maneuvers of the vehicle.

As shown in FIGS. 1 and 2, the hydaulic reaction mechanism B includestwo pairs of opposed pistons 17 which are axially slidably disposedwithin a pair of parallel radial bores 14a formed in an upper endportion of the output shaft 14. The pistons 17 are each applied with ahydraulic reactionpressure in a reaction fluid chamber B₁ which isformed between the valve housing 11 and the upper end portion of outputshaft 14. The hydraulic reaction mechanism B further includes a pair ofradial arms 13b which are integrally formed with a lower end portion ofinput shaft 13 anddisposed between each pair of the pistons 17. Thereaction fluid chamber B₁ is connected to the fluid reservoir R by wayof fluid circuits 25 and 26. The fluid circuit 25 is connected to thefluid circuit 21 through a bypass circuit 27. Disposed between the fluidcircuits 25 and 26 is a flow control valve D of the bypass type which isincorporated with the fluid circuit 21 to control the quantity of fluidunder pressure flowing thereacross from the pump P to the inlet port A₁of rotary valve assembly A and to permit a predetermined small quantityof fluid under pressure supplied thereacross to the fluid circuits 25and 26. A throttle O_(L) is disposed within the bypass circuit 27 tothrottle the flow of fluid supplied therethrough from the fluid circuit21 into the fluid circuit 25, and a throttle O₂ is disposed within thefluid circuit 26to throttle the flow of fluid discharged therethroughinto the fluid circuit 22.

As shown clearly in FIG. 3, the bypass type flow control valve D has avalve housing which is provided with an inlet port D₁ connected to thepump P through the fluid circuit 21, a pair of radially opposedbypassports D₂ each connected to the fluid circuits 25 and 26, and anoutletport D₃ connected to the inlet port A₁ of rotary valve assemblyAthrough the fluid circuit 21. The flow control valve D includes a spool18 slidably disposed within an axial bore in the valve housing, and acompression coil spring 19 arranged to bias the spool 18 upwards. Thespool 18 is formed with a radial hole for providing fluid communicationbetween the ports D₁ and D₃ and formed with a fixed orifice 18a arrangedbetween the port D₁ and the bypass ports D₂. A solenoidrelief valve E isdisposed within the fluid circuit 26 downstream of the throttle O₂ andconnected to an electric control unit G which is connected to acommercially available speed sensor F to produce therefrom an electriccontrol signal indicative of travel speed of the vehicle. Whenappliedwith the control signal, the solenoid relief valve E is energized todecrease the quantity of fluid discharged therethrough into the fluidcircuit 22 in accordance with an increase of the vehicle speed therebyto increase the hydraulic pressure in the reaction fluid chamber B₁.

In operation of the power-assisted steering system, the fluid circuits25 and 26 are supplied with the predetermined small quantity of fluidunder pressure from the bypass ports D₂ of flow control valve D. Whenthe rotary valve assembly A is operated to produce a hydraulic pressurein thefluid circuit 21 in accordance with a steering load acting on thedirigibleroad wheels, the fluid circuit 25 is applied with the hydraulicpressure from the fluid circuit 21 through the throttle O₁ in bypasscircuit 27. Simultaneously, the solenoid relief valve E cooperates withthe throttle O₂ to control the quantity of fluid discharged therethroughinto the fluid circuit 22 in accordance with travel speed of thevehicle. Thus, the reaction fluid chamber B1 of mechanism B is appliedwith a hydraulic reaction pressure from the fluid circuit 25 undercontrol of thethrottles O₁, O₂ and the solenoid flow control valve E.The hydraulic reaction pressure in chamber B₁ is effective to apply areaction force to the driver's steering effort in accordance with travelspeed of the vehicle. In this instance, during stopping of the vehicle,the driver's steering effort will change in relation to the hydraulicpressure in power cylinder C as shown by a characteristic line I in FIG.4. During medium speed travel of the vehicle, the driver's steeringeffortwill change as shown by a characteristic line II in FIG. 4. Duringhigh speed travel of the vehicle, the driver's steering effort willchange as shown by a characteristic line III in FIG. 4.

From the graph of FIG. 4, it will be understood that during stopping ofthevehicle, the hydraulic reaction pressure in fluid chamber B₁ ismaintained in a value T₁ under control of the throttles O₁, O₂ and thesolenoid relief valve E. Similarly, the hydraulic reactionpressure influid chamber B₁ is maintained in a value T₂ during medium speed travelof the vehicle and is also maintained in a value T₃ during high speedtravel of the vehicle. This is useful to enhancestability of thesteering wheel in its neutral position in accordance with travel speedof the vehicle. In FIG. 4, the reference character Pm in FIG.4represents a predetermined pressure level defined by a relief valve (notshown) in the pump P, the reference character T₄ represents a reactionforce caused by the torsion bar 12 in the changeover valve unit, and thereference character T₅ represents a reaction force caused by an increaseof the steering load acting on the dirigible road wheels. In addition,if in the steering system, the bypass circuit 27 with throttle O₁ iseliminated, the characteristic lines I, II, III will be obtained asshown by one dot and dash lines in FIG. 4. If in the steering system,the throttle O₂ is eliminated, the characteristic lines I, II, III willbe obtained as shown by two dots and dash lines in FIG. 4.

The power-assisted steering system may be modified as schematicallyillustrated in FIG. 5, wherein the bypass type flow control valve D isreplaced with an ancillary hydraulic pump P₂ which is coaxially arrangedwith the primary pump P to supply a predetermined small quantity offluid under pressure to the fluid circuit 25. In such a modification, itis preferable that the ancillary pump P₂ is in the form of a smallsizepump mounted on a common drive shaft of the pump P to be driven bytheprime mover of the vehicle. Assuming that the rotary valve assembly Ais maintained in a neutral position during operation of the modifiedsteeringsystem, the auxiliary pump P₂ is driven to maintain thehydraulic reaction pressure in fluid chamber B₁ at the predeterminedlevel, while the pump P is driven in unloaded condition. This is usefulto reducepower consumption of the prime mover for driving the pump Pduring straighttravel of the vehicle.

Having now fully set forth both structure and operation of preferredembodiments of the concept underlying the present invention, variousotherembodiments as well as certain variations and modifications of theembodiments herein shown and described will obviously occur to thoseskilled in the art upon becoming familiar with said underlying concept.Itis to be understood, therefore, that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallyset forth herein.

What is claimed is:
 1. A power-assisted steering system in an automotivevehicle, comprising:a hydraulic pump driven by a prime mover of thevehicle for supply of fluid under pressure; a hydraulic power cylinderoperatively connected to the dirigible road wheels of the vehicle; achangeover valve unit including an input member arranged to be moved bythe driver's steering effort applied thereto, an output memberoperatively connected to said power cylinder, and valve means forcontrolling the supply of fluid under pressure from said pump to saidpower cylinder in response to relative movement between said input andoutput members; a hydraulic reaction mechanism associated with saidchangeover valve unit to apply a reaction force to said input member inaccordance with a hydraulic reaction pressure applied thereto, saidreaction mechanism including a reaction fluid chamber to be applied withthe hydraulic reaction pressure; first flow control valve means disposedwithin a first fluid circuit connecting said pump to said valve means ofsaid changeover valve unit and within a second fluid circuit connectingsaid reaction fluid chamber to a fluid reservoir to permit apredetermined small quantity of fluid under pressure suppliedtherethrough from said pump to said reaction fluid chamber; a firstthrottle disposed within a bypass circuit connecting said first fluidcircuit to said second fluid circuit between said first flow controlvalve means and said reaction fluid chamber; a second throttle disposedwithin said second fluid circuit between said first flow control valvemeans and said fluid reservoir; and second flow control valve meansdisposed within said second fluid circuit downstream of said secondthrottle to decrease the quantity of fluid discharged therethrough intosaid fluid reservoir in accordance with an increase of travel speed ofthe vehicle thereby increasing the hydraulic reaction pressure in saidreaction fluid chamber.
 2. A power-assisted steering system as recitedin claim 1, wherein said reaction mechanism includes means for applyingthe hydraulic reaction pressure to said input member in said reactionfluid chamber.
 3. A power-assisted steering system as recited in claim2, wherein said means for applying the hydraulic reaction pressure tosaid input member comprises a pair of opposed pistons axially slidablydisposed within a radial bore in said output member located in saidreaction fluid chamber, and a radial arm integral with said input memberand disposed between said pistons.
 4. A power-assisted steering systemas recited in claim 2, wherein said means for applying the hydraulicreaction pressure to said input member comprises two pairs of opposedpistons axially slidably disposed within a pair of parallel radial boresin said output member located in said reaction fluid chamber, and a pairof radial arms integral with said input member and disposed between eachpair of said pistons.
 5. A power-assisted steering system as recited inclaim 1, wherein said second flow control valve means is a solenoidrelief valve disposed within said second fluid circuit to be energizedby an electric control signal indicative of travel speed of the vehicle.6. A power-assisted steering system in an automotive vehicle,comprising:a hydraulic pump driven by a prime mover of the vehicle forsupply of fluid under pressure; a hydraulic power cylinder operativelyconnected to the dirigible road wheels of the vehicle; a changeovervalve unit including an input member arranged to be moved by thedriver's steering effort applied thereto, an output member operativelyconnected to said power cylinder, and valve means for controlling thesupply of fluid under pressure from said pump to said power cylinder inresponse to relative movement between said input and output members; ahydraulic reaction mechanism associated with said changeover valve unitto apply a reaction force to said input member in accordance with ahydraulic reaction pressure applied thereto said reaction mechanismincluding a reaction fluid chamber to be applied with the hydraulicreaction pressure; first flow control valve means disposed within afirst fluid circuit connecting said pump to said valve means of saidchangeover valve unit and within a second fluid circuit connecting saidreaction fluid chamber to a fluid reservoir to permit to a predeterminedsmall quantity of fluid under pressure supplied therethrough from saidpump to said reaction fluid chamber; and second flow control valve meansdisposed within said second fluid circuit between said first flowcontrol valve means and said fluid reservoir to decrease the quantity offluid discharged therethrough into said fluid reservoir in accordancewith an increase of travel speed of the vehicle thereby increasing thehydraulic reaction pressure in said reaction fluid chamber.
 7. Apower-assisted steering system as recited in claim 6, further comprisinga throttle disposed within said second fluid circuit between said firstflow control valve means and said second flow control valve means tothrottle the flow of fluid passing therethrough from said reaction fluidchamber to said second flow control valve means.
 8. A power-assistedsteering system as recited in claim 6, further comprising a throttledisposed within a bypass circuit connecting said first fluid circuit tosaid second fluid circuit between said first flow control valve meansand said reaction fluid chamber to throttle the flow of fluid passingtherethrough from said first fluid circuit to said second fluid circuit.